The present invention relates to a general X-ray and electron imaging device, particularly useful for verification, control and optimization of radiation treatment of cancer as well as for applications like diagnostic X-rays, non-destructive testing and screening of containers and vehicles in airports and in customs. More particularly it relates to a detector system with high efficiency over a wide range of photon and electron energies, from diagnostic X-rays starting from the low energies of a few keV all the way up to a hundred MeV, i.e. energies that are of interest and used in radiation therapy or for imaging of large and/or dense objects.
Real time electronic detectors have during the last 30 years revolutionized many areas of X-ray imaging. This includes diagnostic modalities like computed tomography for detailed imaging of the human head and body as well as image intensifiers and video techniques for imaging of the cardiovascular system and for airport security. There are several advantages with real time electronic detectors including improved detection efficiency, wider dynamic range and instantaneous response. Digital images also allow immediate display, electronic storage, diagnosis through telecommunication and computer-aided detection, on-one image enhancement and diagnosis. In spite of the obvious advantages with digital imaging it has turned out to be very hard to replace current film-screen combinations in applications demanding high spatial resolution over large areas, in particular when constraints like high tolerance to radiation damage and reasonable cost are added. Despite its advantages film has a number of disadvantages such as low efficiency, limited dynamic range, noise and the need for chemical development.
The working principle of the present range of electronic detectors is that photons transmitted through the irradiated object are converted to electrons through electromagnetic interactions. Those electrons are in some devices collected directly by dedicated sensors or they are guided through some fluorescent material where secondary light is created and this light is in turn detected by a sensor like e.g. a CCD. In imaging devices for higher X-ray energies, a special converter is added in front of the detector to increase the probability for electromagnetic interaction of the X-rays. This is needed to increase the efficiency of the devices since higher energy X-rays are much more penetrating and would otherwise pass the detector undetected. The converter is usually made as a thin plate of some heavy metal like copper or iron, but molybdenum, chromiun or tungsten are equally suitable. In principle any material could be used, but the efficiency of the device will increase with increasing atomic number. Thus, an atomic number greater than 20 is preferable.
For the purpose of this application the term xe2x80x9celectromagnetic interactionsxe2x80x9d should be taken to encompass all physical interactions between photons and matter that causes generation of at least an electron, i.e. via Compton effect, pair-production or photo electric effect.
The term xe2x80x9cconversionxe2x80x9d is meant to encompass any process involving a photon, wherein some or all of the energy of that photon is transferred to some other corpuscle and wherein a free electron is produced as a result of said energy transfer. Thus, a xe2x80x9cconverterxe2x80x9d is any device capable of producing this effect. It could simply be a gas enclosed in a volume, wherein incident photons interact with the gas in the photo-electric effect to produce electrons. It can also be a sheet or other type of structure of a solid material, in which electrons are generated via the Compton effect or by pair production (electronxe2x80x94positron generation).
xe2x80x9cAmplificationxe2x80x9d is to be construed as a process where one electron interacts with atoms or molecules of a gas thereby causing ionization thereof to produce a plurality of electrons and xe2x80x9cholesxe2x80x9d (positive gas ions). Thus, xe2x80x9camplificationxe2x80x9d is meant to encompass both primary ionization regardless of whether there is an electric field present or not, as well as the well known avalanche fenomenon that occurs in electric fields of the order of 104 V/m or more.
Thus, an xe2x80x9camplifierxe2x80x9d will encompass any structure that causes such xe2x80x9camplificationxe2x80x9d it could e.g. simply be a gas enclosed in a volume where incident electrons will interact with the gas, or a more complex device where an electric field is generated.
Radiation therapy and surgery remain the main modalities for cancer cure in the industrialized world. Radiation therapy is used for more than half of the new cancers with permanent cradication of the tumor without severe complications in more than half of the cases. The radiation dose is delivered to the patient in different fractions, one fraction a day over a period of a couple of weeks. Alignment of the radiation field relative to the tumor is of paramount importance. The alignment has to be particularly accurate when intensity modulation is used and the tumor is close to sensitive organs like the spinal cord. Positioning errors should by no means exceed 2 to 5 mm depending on treatment site. Monitoring and controlling the treatment with a detector behind the patient is usually referred to as portal imaging. More recently, it has been shown that a correction of the patient set-up using the information from an Electronic Portal Imaging Device (EPID) increases the probability of a complication free tumor cure in the order of 10%. However, as already indicated, film still remains the most common tool for verification and quality control of the treatment and is used in more than 90% of the cases. The EPID""s has proven valuable since digital images allow electronic storage and processing of the data. They also in principle enable an on-line control and verification of the treatment even if this is difficult because of the low efficiency of the present EPID""s and the corresponding relatively long times for data acquisition. They also facilitate an adaptive real time control during the course of delivery of the different fractions of the treatment. In portal imaging, it is obvious that the detectors need to be highly radiation tolerant and this is a severe constraint one has to take into account when designing the detector.
There are two main types of EPID systems available commercially today: One is a mirror-based video system and the second is an electronically scanned liquid-ionization chamber system. In both cases, the incident photons are converted to electrons with an efficiency of about 5%-8% through interactions in a metal plate, typically 1.5 mm of copper. If the metal is made thicker, scattering of the electrons in secondary reactions is becoming a problem and electrons will stop in the metal. The typical range for 1 MeV electrons in Cu is less than 0.7 mm. This range is approximately proportional to the energy of the electrons. This puts a fundamental limit on the obtainable efficiency for these devices. Both approaches have proven valuable in localizing the patient in the radiation field and verification of the radiation therapy. A major drawback is that the contrast and quality of the resulting images only makes the bone structure visible and not internal organs and the tumor itself, the exact position of these organs remains unknown. The only way of being sure about these positions would be diagnostic X-ray images taken with the patient in the actual treatment position, without movement of the patient and right before the actual treatment starts since any movement would cause change in position of the internal organs. Unfortunately existing EPID""s are almost insensitive to X-rays of diagnostic energies.
The main specific drawback with the video system is its low efficiency due to loss of photons in the process of de-magnifying the fluorescent screen through a mirror, lens or fiber optic taper to the camera. This efficiency is in fact less than 0.01%. Another problem is the inherent bulkiness of the system that may hamper patient set-up and make them difficult to use in machines with beam stoppers to stop the radiation beam after passing the patient.
In the liquid-ionization chamber the pixels are scanned by a switched high voltage one row at a time and the currents from the pixels are read out by a row of 256 electrometers, the whole detector consist of an array of 256xc3x97256 pixels with a spacing of 1.27 mm. This generates a current of typically 50 pA and the noise is around 0.5 pA. The liquid is integrating the created charge for around 0.5 s and it takes around 5 s to get an image. The drop in efficiency due to the scanning is thus a factor 10. Limitations are long-time stability of the ultra-clean liquid and pick-up due to the high-voltage switching.
The most promising emerging EPID seems to be amorphous silicon arrays. They have been developed since around 1990 but are not yet a commercial product. Advantages compared to the video system are much better optical coupling (around 50%) between the fluorescent screen and the detector since the array is positioned in close proximity to the screen and there is no demagnification. Each pixel is controlled by an a-Si transistor, one row of pixels is gated at a time, and the accumulated charge is amplified by a row of preamplifiers and digitized by a 12 bit ADC. Amorphous silicon has the advantage that it can be deposited over large areas but is not ideal for fabrication of transistors; the ON resistance usually exceeds mega-ohms and this slows down the readout of the charge. In spite of enormous investments from the flat-panel display industry it is not trivial to manufacture large arrays without defects and the cost for a large instrumented a-Si array for X-ray imaging (xcx9c25xc3x9725 cm2 size) is very high. The efficiency is also for this device limited by the fact that only 6%-8% of the incident photons interacts at all in the detector.
The trend in radiation therapy is towards conformal intensity modulated treatments and hyperfractionation that reduces the dose per treatment field. This increases the demands on the EPID in terms of efficiency, high quality image for alignment checks should be obtained at dose levels of 0.01 Gy corresponding to an image acquisition time of 0.25 s at a dose rate of 2 Gy per minute. For a total dose for the field of 1 Gy this means the treatment maybe aborted at radiation levels of less than 1% of the single field dose in case of misalignment. The intended set-up may be documented through either a simulator or a digitally reconstructed radiograph (DRR), which has been reconstructed for a certain beam set-up using computed tomography. Potentially this will enable computer-aided on-line detection of misalignments of the radiation field.
If one compares the EPID to for example an upgrade in accelerator equipment for the treatment unit the cost for an EPID would be less than 0.15 M$ while a new accelerator would cost about 2 M$. Since a portal imager would have very significant impact on estimated benefits for the patient in terms of increased probability of eradicating the tumor, it is in reality a very cost-effective device compared to other investments. If the effect on the outcome of the treatment is 10%, this corresponds to about 1,5 million more patients saved in the U.S. per year.
Thus, there is still a strong need in this field for a detection means that allows an adaptive real time control during the course of delivery of radiation during treatment. In addition, it would be advantageous if the same detection system could be used for both low and high-energy photons, such that for quality control purposes in medical care, a high quality image could be obtained before therapeutic irradiation begins. Furthermore, it would be advantageous if there need be no physical shift or replacement of the detection unit between high and low energy detection, i.e. the detector units should not need to be moved or damaged due to exposure to high energy radiation.
These objects are achieved with a device, method and system as defined in the appended claims.
In particular the present invention in a preferred embodiment concerns detectors comprising a plurality of amplifier and converter stages.
The spatial resolution is determined by the pixel pitch, which will be around 1 mm in the prototype detector, but could be taylored to suit the application in question. This is not a very competitive resolution for diagnostic medical imaging but is sufficiently high for portal imaging. The portal imager according to the invention will also be used as a detector for diagnostic X-rays. It may not be the optimum detector for this task but it will provide valuable additional high-contrast images to correct for internal displacements of sensitive organs as well as the target with the patient in the actual treatment position. To use separate X-ray detectors for all these tasks is impractical. With some modifications the system can also be used for precision dosimetry and current mapping of therapeutic radiation fields. It can thus be used to optimize the dose delivery with different radiation treatment units and techniques.
A particular advantage with a preferred embodiment of the detection system according to the present invention is that it allows the contrast of images produced to be optimized to a high degree and also makes it possible to determine the elemental composition of different parts of the object. This is achieved by the provision of gain control for each individual amplifier in the stack, whereby detection of photons can be discriminated between high and low energies.
Further advantages with the invention are:
1) Possibility of an order of magnitude higher efficiency compared to present detectors for high energy X-rays due to use of multiple conversion layers in combination with efficient collection of the signals from each conversion layer. The integral signals from all the individual layers are detected by one single matrix of sensors.
2) A high signal to noise ratio due to the amplification of the signal in the gas.
3) High radiation resistant since no active electronics need to be directly exposed to the beam, if desired.
4) Very fast parallel read-out enabling acquisition of the whole image matrix in less than 3 ms, if desired.
5) The energy response of the detector can be changed simply by altering the potential on the different electrodes.
6) Rugged design where the amplification is geometrically stable but adjustable.
7) Highly efficient over a wide range of energies. This enables the combination of an detector for diagnostic and therapeutic X-rays in one single device by using a thin entrance window and gas volume on top of the first converter layer. If a diagnostic X-ray tube is inserted above the patient a high contrast diagnostic X-ray image could be obtained right before the treatment starts and thus the exact position of any organs and the tumor itself could be determined.
8) Energy sensitive if desired. This makes it possible to optimize the contrast for any given imaging task and also opens the possibility to determine the elemental composition of the object. This energy sensitivity also enables dual-energy imaging in the sense that it is possible to determine not only the X-ray attenuation in the object but also the different elements the object consist of by comparing images with different weighting of low and high energy X-rays.
9) The invention also offers the possibility to weight the information from X-rays of different energies in such a way that the contrast in the resulting image is optimized for the object of interest.